Recently, in electronic machinery and tools, such as a drive case for a CD-ROM, a related machinery and tools for a personal computer, and a measuring machinery and tools, further smaller size and light weight are tried to be achieved more than before, and as materials for machinery and tools parts, those compatible thereto are required. For the machinery and tools part material, a good anti-fingerprint property, a high formability and workability (hereinafter, formability and workability are collectively referred to as formability), and electric characteristics (a grounding property and a shield property) not affecting the performance of a precision electronic machinery and tools main body, are required.
Conventionally, as electronic machinery and tools part materials, the following are representative. However, none of them can satisfy all of the above-mentioned required qualities.
First, as a material of parts of electronic machinery and tools, a steel sheet (in particular, plated steel sheet) is most common. However, since the specific gravity of steel sheet is high, it is not suited for the trend toward mobile and lighter-weight electronic machinery and tools. Moreover, an aluminum bear material is widely used as well. Although the aluminum bear material has good conductivity, it has a disadvantage in that fingerprints are left on the surface easily at the time of handling (inferior in stain-proofing property). Therefore, an aluminum-coated material, obtained by applying a resin coat on an aluminum sheet, has been proposed. However, although the aluminum-coated material has a good anti-fingerprint property, it has a large electric resistance value in a coated resin layer, so that desired electric characteristics (a grounding property and a shield property) cannot be obtained when it is made into an electronic machinery and tools part.
Further, an aluminum material has a benefit that it has excellent elongation and expanding property in comparison to other metal materials. Therefore, it is suitable to be applied to forced forming. However, when the above-mentioned conventional metal materials, each having a resin coating on the surface, was applied to forced working, there was such a defect that the resin layer could not keep sufficiently the elongation and expanding property of the aluminum material of base material, to cause peeling or cracking of the resin layer, or the like.
Moreover, these aluminum-coated materials include a material having a coated film that is formed on the surface and that is composed of a mixture obtained by adding a metal soap, for improving the lubricant property and the formability, and a conductive powder (1% by weight or less), for improving the welding property, to a water-soluble lubricant high polymer wax as the main component, as disclosed in JP-A-5-320685 (“JP-A” means unexamined published Japanese patent application). However, in terms of the above-mentioned electric characteristics (a grounding property and a shield property) required for recent electronic machinery and tools parts, a target effect cannot be obtained.
Furthermore, Japanese Patent No. 2,133,521 discloses a part of electronic machinery and tools obtained by forming a resin-coated film, containing a lubricating agent, only on one side of an aluminum alloy sheet, and applying forming process to the sheet into a floppy disc drive case, with the coated surface on the outer side, so that electrical ground can be obtained through the surface without the coated film on the inner side. Since the side with the resin-coated film does not have conductivity at all, it cannot satisfy the above-mentioned electric characteristics, and there is a limitation in use.
JP-A-6-240469 discloses a resin layer, on a steel sheet provided with a layer subjected to chromate process, which resin layer comprises one or two or more resin selected from the group consisting of epoxy, alkyd, acrylic, urethane, phenol, melamine, polyvinyl butylal, and polyester resins, and is added a powdery lubricating agent, and a conductive auxiliary agent (particles of one or two or more selected from the group consisting of Cu, Ni, Ag, Al, Zn, Cr, Fe, Co, an alloy thereof, carbon black, and carbon graphite) having a 0.1 to 5 μm average particle size. However, as a result of our testing of the above steel sheet having a resin layer, scattering of conductivity of it was large, and it could not satisfy the grounding property and shield property required.
Moreover, as a conventional coating method, a roll coat has been the mainstream. In particular, a bottom-up method, shown in FIG. 2, is used in most cases. The bottom-up method is, as shown in the figure, a method of storing a coating material (e.g. paint, varnish) in a coater pan, taking up the coating material with a pick-up roll, and transferring the coating material onto an applicator roll, thereby coating on an aluminum sheet. A reverse roller coating, of transferring the coating material by reverse rotation of the applicator roll with respect to the progress direction of the aluminum sheet at that time, is common. However, since the specific gravity of a conductive auxiliary agent of a metal, such as Ni, is much higher than the specific gravity of a coating material, the conductive auxiliary agent is gradually settled in the coater pan, so that a predetermined amount of the conductive auxiliary agent cannot be entered in the resin layer, and thus a desired electric characteristic cannot be obtained.